This invention relates to electrical switches and, more particularly, to non-resettable, pressure-actuated electrical switches having a pressure-sensitive diaphragm and a frangible conductive path that can be irreversibly broken by forces resulting from the pressure-actuated deflection of the diaphragm.
Electrical switches having a pressure-sensitive diaphragm that deflects in response to an applied pressure differential to open or close electrical contacts are used in the monitoring and control of fluid systems. A micro-engineered electrical switch that makes or breaks an electrical contact when the pressure differential across a domed snap-action diaphragm exceeds a certain level is disclosed in U.S. Pat. No. 4,965,415. As the pressure differential decreases, the diaphragm snaps back to its original position, thereby resetting the electrical contacts to their prior make or break condition.
There are, however, situations in which it is undesirable to have the switch reset itself. Certain depth- or pressure-sensitive instrumentation experience a reduction in performance or accuracy as a result of exposure to depths or pressures beyond design limits. The resulting reduction in performance is often subtle and not easily discernible by the user. In some cases, equipment damage could result from such misuse. In such situations, it is often important that uncontrovertible, non-resettable evidence of misuse be available.
There are other situations in which a resettable switch is not desirable. U.S. government regulations covering the export of high technology require that certain electronic devices not operate outside of specified operating limits. In the case of highly accurate digital heading sensors deployed on submarine-towed sonar arrays, government regulations require that sensors sold to certain foreign countries fail once they are used below a specified depth. Such a hard failure is undesirably correctable with a resettable device.
Non-resettable, flow-actuated devices are used to indicate the safety relief of overpressure in fluid flow applications. In U.S. Pat. No. 4,978,947, a rupturable fluid flow indicator is disclosed. The indicator includes an electrical path traversing a weakened portion of a rupturable perforated disk clamped across a fluid flow passageway. When the pressure differential across a related pressure-sensitive safety membrane connected across the passageway between the disk and the fluid exceeds a specified level, the membrane ruptures, thereby diverting some of the fluid into the passageway. The flow pressure of the diverted fluid exerted on the disk causes it to rupture, breaking the electrical path, which can set off an alarm. Although the fluid flow indicator is not resettable and operator intervention is required to replace it, the device relies on fluid flow to break the electrical path. Thus, a passageway for the fluid to flow in must be provided. Furthermore, the perforations in the disk reference the pressure differential to the pressure existing in the passageway, typically atmospheric. The pressure-sensitive membrane, moreover, ruptures only if the external pressure exerted by the fluid is too high; it is not designed to operate when the external pressure is low. The usefulness of the indicator is limited to pipeline fluid flow applications; it is not a self-contained unit with general utility.
For the foregoing reasons, there is a need for a simple, self-contained, general-purpose, pressure-sensitive device to irreversibly break an electrical path when the applied pressure exceeds a specified level.